1. Field of the Invention
The present invention is directed generally to an image forming apparatus capable of removing filming formed on an image carrier and refreshing developer and to an image forming method.
2. Description of the Related Art
It is known that an image forming apparatus, such as a copier or a printer, develops an electrostatic latent image formed on a photoconductor that acts as a latent-image carrier into a visible image with developer supplied from a developing device.
The developed toner image is transferred onto a transfer member and thereafter passed to a fixing device where toner on the toner image is fused and absorbed into a recording medium by heat and pressure. Hence, a copied or printed image is obtained.
Meanwhile, from the photoconductor, from which the toner image has been transferred, the photoconductor foreign materials, such as paper powder and residual toner having not been transferred to the recording medium, are removed by a cleaning device. The photoconductor is further subjected residual-charge neutralization.
The cleaning device includes, as a member that comes into contact with the surface of the photoconductor, a cleaning blade or a cleaning brush, which allows the cleaning device to remove foreign materials from the surface of the photoconductor.
Removing foreign materials with a cleaning device can bring about a problem related to roundness and smaller diameter of toner particles that are employed in recent years to meet increasing demands for higher image quality, and a problem related to filming of toner caused by accumulation of electrostatic discharge products. The former problem is generally caused by accumulation of toner particles in a case where toner particles slip through the blade or brush.
A general cause of the latter problem is described below.
In recent years, electrostatic charger that includes an electrostatic charging member and performs electrostatic charging with the electrostatic charging member placed on or near a photoconductor is widely used as an electrostatic charging unit in an electrophotographic-image process. Although such an electrostatic charger advantageously produces a small amount of ozone by application of an alternating-current (AC) bias voltage to the electrostatic charging member as compared with a corona charger, the electrostatic charger disadvantageously produces adverse effects due to discharge products resulting from the application of the AC bias voltage. More specifically, the discharge products adhere to a surface of a photoconductor to form what is called photoconductor filming, thereby decreasing electrical resistance of the surface of the photoconductor and decreasing resolution of the latent image. As a result, a phenomenon, such as image blur and/or image deletion, is more likely to occur. The filming becomes more pronounced as the AC bias voltage increases. Such photoconductor filming disadvantageously lowers evenness of the photoconductor surface and hence induces defective cleaning of the photoconductor, thereby making quality change with time less stable.
To this end, a method of scraping off a photoconductor during image forming operation by using developer supplied from a developing device has conventionally been proposed. An example of such a technique is disclosed in Japanese Patent Application Laid-open No. H11-52789.
According to the configuration disclosed in Japanese Patent Application Laid-open No. H11-52789, when it is determined that filming needs to be removed, the photoconductor is driven intermittently or peripheral speed of the photoconductor is set to a speed lower than normal-image-forming speed, and the developer supplied from the developing device is caused to adhere to the photoconductor so that the filming is removed by scraping off the developer adhering to the photoconductor using a cleaning member. In this technique, the peripheral speed of the photoconductor is reduced to a range of 0 to 0.1 times an initial peripheral speed while a peripheral speed of a magnetic roller of the developing device is set to 2.5 times an initial peripheral speed.
An example of another method is disclosed in Japanese Patent Application Laid-open No. 2007-271871. According to this technique, an image, such as a solid image, that consumes a relatively large amount of developer is formed on a photoconductor and developed in a manner that stops toner supply to a developing device so that developer in the developing device is replaced to achieve developer refreshing.
An example of still another method is disclosed in Japanese Patent Application Laid-open No. 2009-192821. This technique is generally directed to elongated, continuous paper rather than to a sheet of recording paper, and removes filming by applying a bias voltage of opposite polarity to polarity of a developing bias voltage when “form printing”, in which a same print pattern is constantly printed, is performed.
Meanwhile, filming can be formed not only with one-component developer that contains only toner but also with two-component developer that contains toner and carrier in a mixed manner. In particular, when a printout of which ratio of a toner-applied area to an area of the printout is relatively small is produced with use of two-component developer, toner in the developer is consumed little in some cases. This can enhance electrostatic buildup by friction between the toner and carrier, causing the toner to bear a relatively large amount of electric charges. Accordingly, when a small-diameter toner is employed, an amount of electrical charges per unit weight of the toner is increased, which increases electrostatic attraction of the toner to carrier. Consequently, a relatively large amount of small-diameter toner particles are contained in the developer.
When an amount of toner adhering to the carrier increases, the toner is less likely to be delivered onto an electrostatic latent image, which can result in decrease in image density.
Furthermore, toner particles charged in opposite polarities are more likely to repel one other, causing apparent bulk density of the developer to differ from its actual bulk density. This can bring about such a disadvantage that, in a situation where toner concentration is monitored by using a permeability sensor, the high apparent bulk density of the toner misleads determination of toner concentration such that the toner concentration is falsely determined as being sufficiently high even when toner density is low. This can lead to additional disadvantages, such as failure of toner supply and production of friction by carrier particles on the surface of the photoconductor resulting from insufficient toner particles.
Among the techniques for removing filming discussed above, the technique of moving a photoconductor intermittently to supply developer to thereby scrape off filming is disadvantageous in that an increase in friction force at start of traveling of the photoconductor can increase damage to the photoconductor. The technique that involves application of a bias voltage of opposite polarity when a two-component developer is used for scraping off the filming is disadvantageous in that an amount of scattering carrier particles increases, causing a developing-roller locking phenomenon to be likely to occur.
The technique related to developer refreshing is disadvantageous in requiring another operation of replacing already-supplied developer with new developer because, according to the technique, only consumption of the already-supplied developer is performed without supplying new toner.
Each of these techniques discussed above is predicated that removal of filming and developer refreshing are independently performed and that toner to be used has already been degraded over time.
To remove filming, toner that is not degraded over time yet in terms of electrostatic charging characteristics and surface characteristics is desirably used; however, filming removal according to the techniques discussed above is performed by using already-supplied toner and therefore disadvantageous in being unlikely to be efficient in filming removal nor good in developer refreshing performance. The method in the technique discussed above is also disadvantageous in re-configuring a cycle for filming removal and therefore requiring a considerable period of time for operations other than image forming.
Setting the peripheral speed of the photoconductor to be lower than normal-image-forming speed or stopping the photoconductor as with the technique disclosed in Japanese Patent Application Laid-open No. H11-52789 is disadvantageous in promoting filming contrary to expectation.